U.S. President Barack Obama and Russian President Vladimir Putin spoke about the crisis in Ukraine on Monday, the White House said.

New White House press secretary Josh Earnest confirmed the call to reporters at the White House's daily press briefing. He said Obama urged Putin to stop supporting pro-Russian separatists in southeastern Ukraine, where the U.S. says Russia has sent tanks and other arms in recent weeks. Obama warned that while a diplomatic solution was still possible, Russia would face additional "costs" if it didn't work to de-escalate the situation.

Earnest confirmed the call after initially saying he had no calls to read out to the press at the briefing. Earnest only announced the call when he was asked — after the Kremlin had disclosed it in a statement. The Kremlin said the two sides vaguely discussed the "prospects of resolving the situation."

The Obama-Putin call came as pro-Russia separatists backed a ceasefire proposed by newly installed Ukrainian President Petro Poroshenko that will last until Friday.